As the aggregate data rates in optical fiber communication systems continue to increase, the required output powers from fiber optic amplifiers also increase. In addition, as fibers are installed to carry CATV video channels to the home, unprecedented power levels will be required to overcome the massive splitting losses and high power demands of analog communication systems. Remotely pumped communication systems also require high power fiber amplifiers. Current amplifiers are already pushing against the limits imposed by facet damage of conventional single-transverse-mode semiconductor pump lasers. Thus, it is necessary to develop fiber optic amplifiers that are capable of providing high output power.
A promising approach utilizes cladding-pumped (CP) Fiber amplifiers. In this type of amplifier, multi-transverse-mode pump light is injected into the inner cladding of a CP fiber while single-transverse-mode signal light is injected into the core of the fiber. The core of the CP fiber is typically doped with a rare-earth dopant that absorbs the pump light and then serves to amplify the signal. Often the rare-earth-doped core contains both erbium and ytterbium. The pump light (typically between 900-980 nm) is absorbed by the ytterbium ions which then transfer their excitation to the erbium ions. The excited erbium ions may then amplify a 1.5 .mu.m signal. Various schemes can be used to achieve the required multimode/single mode coupling. See, for instance, U.S. Pat. No. 5,864,644 filed Jul. 31, 1997 by DiGiovanni and Stentz, incorporated herein by reference.
The efficient conversion of input pump power to output signal power is extremely important for a power amplifier. A power amplifier with low conversion efficiency will need to dissipate more heat from its pump diodes and thermoelectric coolers, from the amplifier fiber itself, and from the amplifier package on the whole. In addition, the reliability of pump diodes generally decrease with increasing output power. Therefore, the reliability of an amplifier is typically compromised by poor conversion efficiency. In view of these facts, it is evident that it is highly desirable to have available a fiber optic power amplifier having high conversion efficiency. This application discloses such an amplifier.